Add the lemon juice, egg yolks, granulated sugar, and lemon zest to a medium saucepan.
Whisk together well.
Cook over medium-low heat for about 15-20 minutes, whisking frequently, until the mixture is thick enough to coat the back of a spoon.
Remove it from the heat.
Whisk in the cubed butter a few cubes at a time until well-combined.
Carefully pour the lemon curd into a heat-safe bowl.
Press a piece of plastic wrap to the surface of the lemon curd.
Place the lemon curd in the refrigerator and chill until completely cooled.
Notes
Lemon juice: Please don't use bottled lemon juice in this recipe. Bottled lemon juice can have an off taste to it.
Sugar: Feel free to adjust the amount of sugar to your liking. I like the lemon curd to be on the tarter side, but you can adjust the sugar if you like it sweeter.
Lemon zest: The lemon zest helps to add more lemon flavor to the lemon curd.
Butter: If you prefer to use unsalted butter, add 1/4 teaspoon of salt along with the sugar.